It is well known that a movable part of an injection molding machine is driven by an AC servo motor. A description will now be briefly given of an instance of an injection molding machine, in which a movable part is driven by an AC servo motor, with reference to a schematic view shown in FIG. 2.
Roughly speaking, the injection molding machine is composed of a clamping part 100 and an injection part 200. In the clamping part 100, a fixed platen 101 fixed to a base of the injection molding machine is connected to a rear platen 103 with four pieces of tie bars 104, a movable platen 102 is slidably fitted to and is guided by the tie bars 104, a mold 105 is mounted on the movable platen 102 and the fixed platen 101, and a toggle mechanism 106 is provided between the rear platen 103 and the movable platen 102.
The toggle mechanism 106 is driven through a ball screw/nut mechanism 107 by an AC servo motor Mc for clamping and then moves the movable platen 102 to open or close the mold 105 for clamping. Reference symbol Me denotes an AC servo motor for ejecting, which drives an eject mechanism 108 to eject a molded product from the movable-side mold 105. Incidentally, reference symbol M1 is a motor for mold-thickness adjustment, which moves the rear platen 103 through a pulley, a belt or the like to a position where predetermined clamping force can be exerted according to the thickness of the mold 105.
The injection part 200 is provided with a pusher plate 202 movable along guide bars provided between a front plate 201 and a rear plate 203, and a heating cylinder 204 is mounted on the front plate 201. A screw 205 is inserted into the heating cylinder 204 and is mounted on the pusher plate 202 so as to be capable of being freely revolved, while being incapable of being moved in an axial direction. Reference symbol Mm denotes an AC servo motor for metering, which revolves the screw 205 through the pulley, the belt or the like. Reference symbol Ml denotes an AC servo motor for injection, which moves the pusher plate 202 through the pulley, the belt and the ball screw/nut mechanism 206 and then moves the screw 205 in the axial direction to perform injection and hold-pressure. Further, back pressure at the time of metering is controlled also by the servo motor Ml for injection.
Reference symbol M2 denotes a motor for nozzle touch, which drives a ball screw/nut mechanism 207 and then moves the whole injection part 200 to cause a nozzle provided on the end of the heating cylinder 204 to come into contact with or be separated from the mold 105 mounted on the fixed platen 101.
In a mode of controlling drive of the AC servo motors Mc, Me, Mm and Ml which drive the movable parts (the movable platen, the eject mechanism and the revolution and axial movement of the screw) of the injection molding machine as described above, positional control is made for the movable platen, the eject mechanism and the axial movement of the screw. For such positional control, a position/speed detector is mounted on the AC servo motors Mc, Me and Ml (incidentally, these servo motors are shown by a reference numeral 4 in FIG. 3); a position control section 10 performs control of speed loop on the basis of a position command and a position feedback signal and finds a speed command, a speed control section 11 performs control of the speed loop on the basis of the speed command and a speed feedback signal and finds a torque command; a current control section 12 performs control of the current loop for each of three phases in response to the torque command and finds a voltage command corresponding to current to be supplied; and the AC servo motor is driven through a power amplifier such as an inverter on the basis of the command voltage, as shown in FIG. 3. On the other hand, for the AC servo motor Mm which revolves the screw, the control of speed loop is made without positional control (the speed command is directly inputted without providing any position control section 10 as in FIG. 3).
FIG. 4 is a block diagram of a current control mode heretofore in use for the AC servo motor. On the basis of the torque command issued from a speed loop and a rotor phase .theta. from a rotor phase detector provided in the AC servo motor, multiplying sin .theta. by the torque command gives a U-phase current command in the U-phase (incidentally, the rotor phase is based on a U-phase); multiplying sin(.theta.+2.pi./3) shifted by 120 degrees in phase by the torque command a V-phase current command in a V-phase, and multiplying sin(.theta.-2.pi./3), further shifted by 120 degrees in phase, by the torque command gives a W-phase current command in a W-phase. Then, in each phase, control of the current loop in integral-plus-proportional control is made on the basis of the current command and the current feedback signal of each phase to find a voltage command (PWM command), and the servo motor is driven through a servo amplifier such as an inverter. Incidentally, in each phase current loop in FIG. 4, reference symbol K1 denotes an integrating gain, K2 is a proportional gain, R is a wire-wound resistance of the servo motor, L is its inductance. Further, reference symbol s in each phase current loop is a Laplace operator.
In the current control mode as described above, since an AC current frequency (i.e., an AC current frequency produced by the inverter) increases in proportion to an increase of the revolving speed of the servo motor, a reduction of gain and a phase lag occur based on frequency characteristics of a control system. Therefore, a power factor is degraded to present problems such as an increase of drive current and a reduction of the maximum torque.
On the other hand, the injection molding machine is designed to repeat a molding cycle continuously, and an increase of speed in the molding cycle is preferably required for the achievement of higher production efficiency. To increase the speed in the molding cycle, it is necessary to drive at higher speeds the AC servo motor which drives each movable part of the injection molding machine. However, high-speed operation causes the increase of drive current as described above. As a result, the motor tends to give off a larger amount of heat, so that the increasing drive current becomes an obstacle to the increase of the speed in the molding cycle. Further, for increasing the speed in the molding cycle, high torque is needed, however, as the reduction of the maximum torque occurs as described above, problems arise also in this point.